NEW YORK (Reuters Health) - Children born to relatively older mothers or fathers may have a higher risk of autism than those with younger parents, a new study finds.

In a study of 7.5 million births in California between 1989 and 2002, researchers at the state's health department found that a child's risk of developing autism increased along with the age of the parents.

For each 10-year increase in a mother's age between the ages of 20 and 40, the risk of her child developing autism climbed by 38 percent -- with the father's age and other factors, like race and parents' education, taken into account.

Similarly, each 10-year increase in a father's age between the ages of 20 and 60 was associated with a 22-percent increase in autism risk.

The findings, reported in the American Journal of Epidemiology, add to a conflicting body of research on what role, if any, parents' age plays in autism development. Past studies have either found that older age may increase the risk, or has no impact at all.

Autism spectrum disorders include several developmental brain disorders that, to varying degrees, hinder a person's ability to communicate and interact socially. The precise causes of autism are not fully understood, though researchers believe that genetic susceptibility plays a key role.

These latest findings suggest, but do not prove, that older age in parents may be an additional risk factor.

"The big problem," lead researcher Dr. Judith K. Grether told Reuters Health in an email, "is that we don't know what factors explain the link."

On one hand, there are potential biological mechanisms by which parents' ages could affect fetal brain development, and thereby the risk of later autism, according to Grether, of the California Department of Public Health.

Hormonal factors in the womb, or a greater number of genetic flaws in older eggs or sperm, for example, could be involved.

"Another possibility," Grether said, "is simply that people who already carry a predisposition -- probably genetic -- for having a child with autism tend to start having children later in life."

If that's the case, then older parental age, per se, would not be a risk factor for autism. However, Grether also pointed out that the two possible explanations are not mutually exclusive, and both could be at work.

The findings are based on data for more than 7.5 million children born in California between 1989 and 2002. The researchers identified 23,311 children who received state-sponsored services for autism.

The large majority of children born to older parents did not develop autism. For example, among children born to mothers ages 40 to 44, 826 developed autism, while nearly 150,000 did not. But when compared with children born to mothers between the ages of 25 and 29, their risk of developing autism was 84 percent higher.

More research is needed to understand why this relationship exists, according to Grether. Until that work is done, she said, "we don't have good advice to give to parents."

A number of studies have shown that autism diagnosis have shot up over the past two decades, for reasons that are not clear. It's possible, according to Grether, that the concomitant trend toward delaying childbirth could have contributed to that rise.

But even if older parental age is a factor, she said, it would be a "relatively minor one."